Speculative design is a subsidiary field of critical design practice. It generally involves developing scenarios based on a central object, often a prototype. Because it is concerned with alternative present and future states, many acknowledge the potential of speculative design for raising critical discussion and public engagement on science, technology, and society. In this article, we ask how the analogy of speculative design to thought experiments highlights or problematizes certain aspects of speculative design. Building on the work of Anthony Dunne and Fiona Raby, we answer this question through a comparative conceptual analysis of speculative design and thought experimentation as they are understood in their original contexts. We go on to create a theoretical framework applied to four cases: Metamorphism; Walden, a game; The Substitute; and GoatMan. We argue that understanding speculative design by analogy to thought experiments could help design practitioners better achieve the aim of public engagement, enabling a more inclusive and nuanced discussion about the form and style of speculative design.